The invention relates generally to diagnostic imaging and, more particularly, to an apparatus for calibrating an x-ray tube.
X-ray systems typically include an x-ray tube, a detector, and a support structure for the x-ray tube and the detector. In operation, an imaging table, on which an object is positioned, is located between the x-ray tube and the detector. The x-ray tube typically emits radiation, such as x-rays, toward the object. The radiation typically passes through the object on the imaging table and impinges on the detector. As radiation passes through the object, internal structures of the object cause spatial variances in the radiation received at the detector. The detector then emits data received, and the system translates the radiation variances into an image, which may be used to evaluate the internal structure of the object. One skilled in the art will recognize that the object may include, but is not limited to, a patient in a medical imaging procedure and an inanimate object as in, for instance, a package in an x-ray scanner or computed tomography (CT) package scanner.
X-ray tubes include a rotating anode structure for the purpose of distributing the heat generated at a focal spot. The anode is typically rotated by an induction motor having a cylindrical rotor built into a cantilevered axle that supports a disc-shaped anode target and an iron stator structure with copper windings that surrounds an elongated neck of the x-ray tube. The rotor of the rotating anode assembly is driven by the stator.
An x-ray tube cathode provides an electron beam that is accelerated using a high voltage applied across a cathode-to-anode vacuum gap to produce x-rays upon impact with a target track of the anode. The area where the electron beam impacts the target track is often referred to as the focal spot. Typically, the cathode includes one or more cylindrical or flat resistive filaments positioned within a cup for providing electron beams to create a high-power, large focal spot or a high-resolution, small focal spot, as examples. Typically, an electrical current is passed through the resistive elements, thus causing the resistive elements to increase in temperature and emit electrons when in a vacuum.
Imaging applications may be designed that include real-time control of focal spot size (length and width) and position on the target track. The position of the focal spot may be kept at the same track location (ignoring track rotation) or dynamically deflected view-by-view between two or three or more positions. Focal spot control is enabled via electrodes surrounding the filament within the cathode structure. Changes in current (mA) and voltage (kVp) to the cathode filaments affect the position and size of the focal spot.
According to one example, to compensate for current and voltage adjustments, electrode voltages within the cathode are adjusted to achieve a desired or targeted focal spot size and position. According to another example, focal spot size and position may be controlled using magnetic lenses (dipole, quadrupole, multipole) instead of or additional to electrostatic control as described with respect to the electrode voltages. Such adjustments may occur at the start of the scan (dependent upon user selection of mA and kVp) or during an exam (e.g., mA adjustment during the exam). A mapping, referred to as a cathode transfer function, is used to determine the requisite values for the electrode voltages for a targeted focal spot size, deflection distance, kVp and mA.
Due to component manufacturing variability, a cathode transfer function is typically determined for each tube and generator combination to achieve the targeted focal spot sizes and positions (within a predetermined tolerance) for a plurality of currents and voltages. The cathode transfer function determined for a particular tube using one generator, however, may cause the tube to exceed focal spot tolerances when the particular tube is coupled to another generator. For example, a cathode transfer function determined using a generator during a manufacturing process of the x-ray tube may be different from a cathode transfer function of the same x-ray tube using a generator of an imaging system into which the x-ray tube is to be installed.
Therefore, it would be desirable to design an apparatus and method capable of determining the cathode transfer function of an x-ray tube particular to the imaging system into which the x-ray tube is to be installed.